(1) Field of the Invention
This invention relates to an electrophotographic photosensitive material used in an image forming apparatus, and more specifically, relates to an electrophotographic photosensitive material capable of providing an image forming apparatus which can prevent the occurrence of a transverse black stripe at the tip of a transfer paper and can form an image having an excellent quality at a low cost.
(2) Description of the Prior Art
In the image formation according to electrophotography, a photosensitive material is charged uniformly, the image is exposed to form an electrostatic latent image, the latent image is developed with a toner, the toner image is transferred from the photosensitive material to a transfer paper and the toner image on the transfer paper is fixed to form an image.
Methods of development to be used include a two-component type magnetic developing method using a mixture of a toner and a magnetic carrier, a one-component type magnetic development method using a one component toner containing a magnetic powder, and a non-magnetic one-component type development method using a non-magnetic one-component type toner. In view of the cost of the developer, the cost of the developing apparatus, and the simplicity of the operation, the non-magnetic one-component type developing method is best.
In this non-magnetic one-component type development method, a charged thin layer of toner formed on a develping roller composed of an electroconductive elastomer roller is contacted with the surface of a photosensitive material having an electrostatic latent image to form a toner image. If the polarity of the electrostatic latent image (electric charge in a dark portion) is the same as the polarity of the toner, a negative image is formed by reversal development. When these polarities are different, a positive image is formed.
According to a development method using the non-magnetic one-component type toner, a fixed bias voltage is applied to the developing roller to maintain a proper image density and simultaneously to prevent fogging. For example, when a reversal development is carried out by using a positively charged type photosensitive material and a positively charged non-magnetic one-component type toner, an electric voltage having a positive polarity and having about 0.2 to 0.8 times the surface electric potential (potential in the dark portion) is applied to a developing roller.
However, at the beginning of operation of the image forming apparatus, the surface potential of the photosensitive material is zero. Thus, since the toner on the developing roller adheres to the surface of the photosensitive material, a negative voltage is applied to the developing roller to prevent the initial adhesion of the toner, the approaching of the tip of the transfer paper to the photosensitive material is sensed, and the voltage to be applied to the developing roller is switched from a negative voltage to a positive predetermined bias voltage.
However, when a negative voltage is applied to the developing roller at the time of beginning of the operation, an initial surface potential of the photosensitive material is decreased and a defect will be developed in that a transverse black stripe is generated at the tip of the transfer paper. This is presumably because an electric charge is injected from the developing roller to the photosensitive material.
To prevent this defect, means of raising the voltage stepwise are employed at the time of changing the voltage to be applied to the developing roller from a negative voltage to a positive voltage. The provision of such a controlling means makes the apparatus complex and elevates the cost, and is not preferred because the time of starting the formation of an image is delayed.